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Quantifying the Impact of Intra-Prostatic Calcifications and Seed Orientation in Low-Dose Rate Prostate Brachytherapy Using Monte-Carlo Dose Calculations

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C Collins Fekete

C-A Collins Fekete*, M Plamondon, A Martin, E Vigneault, L Beaulieu, Centre Hospitalier Universitaire de Quebec, Quebec, QC

SU-C-500-1 Sunday 1:00PM - 1:55PM Room: 500 Ballroom

Purpose: To quantify the impact intra-prostatic calcifications and seed orientation in prostate brachytherapy using Monte-Carlo (MC) dose calculations.

Methods: MC simulations were made with ALGEBRA (based on Geant4 C/C++ toolkit v4.9.6.p01) to simulate dose deposition in 1mm³ voxelized geometries using post-planning exported DICOMRT files. Materials are assigned from the physicians contours, based on TG186 recommendations, and electron density from CT number. SelectSeed and OncoSeed6711 seeds are modeled and place in the CT geometries. 3D dose maps, isodoses, DVHs and dosimetric indices for each structure are calculated. Orientation study: Orientations are obtained using automated seed detection from five fluoroscopic projections. Five dose algorithms are compared: TG43 1D(1), TG43 2D (parallel to the CT scans (2)/ with orientations (3)) and MC (parallel (4) /with orientations (5)). Calcifications study: Breast calcification definition is used to define the prostate calcification. 6 different scenarios were tested: water-based MC with (1) and without (2) seeds fully modeled, 2) water-based prostate MC with densities from the CT and calcification areas with (3) and without seeds (4) and full heterogeneities with (5) and without seeds (6).

Results: Orientation study: Calculations on 296 clinical geometries indicate no significant difference between parallel and oriented seeds for prostate and urethra but an average difference of 3% (min-max = X-Y%) for rectum and bladder between TG43 and MC. Calcification study: 42 patients (out of 296) with observable calcifications were analyzed. We measure, large differences relative to TG43, up to -27(-28)% for D90 (V100) when all materials are defined and seeds modeled, and up to -25(-22)% when only the calcified areas are considered.

Conclusion: Large patient cohorts have been analyzed to extract the (previously neglected) effect of seed orientations and calcification. Significant discrepancies relative to TG-43 have been outline. Correlation to toxicities and survival will be studied in a forthcoming study.

Funding Support, Disclosures, and Conflict of Interest: NSERC - Natural Sciences and Engineering Research Council of Canada

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